Glutamate is known to be the principle excitatory neurotransmitter throughout the vertebrate central nervous system. It is implicated as the transmitter used by photoreceptors and bipolar cells of the vertebrate retina. However, it has been difficult to distinguish between the action of glutamate and other excitatory amino acids. The Principle Investigator has developed a fluorometric assay for glutamate which exploits the activity of glutamate dehydrogenase of produce NADH in a rate proportional to the glutamate concentration. The production of NADH is monitored flurometrically, and provides a quantifiable measure of the release of glutamate from individual retinal neurons. this assays, coupled with patch pipette recordings of the cells, will be used to examine the release of glutamate from isolated goldfish photoreceptors and bipolar cells. For the bipolar cells, the experiments will examine whether glutamate is the transmitter used. For the photoreceptors, the calcium-dependence of release will be measured, any calcium-independent release will be examine to learn if it is using the same transport mechanism as the high-affinity uptake, and the time course and light modulation of glutamate release form rod and cone photoreceptors will be compared. These experiments will provide a determination of the mechanisms which control the transfer and shaping of visual signals from photoreceptors to bipolar cells. This information will aid in understanding the normal action of these retinal neurons in the processing and transmission of visual information. Glutamate action has been implanted in many neuronal functions, including a possible role in plasticity. Also, glutamate-induced neurotoxicity is considered a significant effect of neuronal ischemia. These experiments will examine how central nervous system neurons control the release of this neurotransmitter, and will provide valuable insight into the normal operation of glutamatergic neurons.